Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
  • H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/02—Structural details or components not essential to laser action
  • H01S5/0201—Separation of the wafer into individual elements, e.g. by dicing, cleaving, etching or directly during growth
  • H01S5/0202—Cleaving
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/1053—Comprising an active region having a varying composition or cross-section in a specific direction
  • H01S5/1064—Comprising an active region having a varying composition or cross-section in a specific direction varying width along the optical axis
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/16—Window-type lasers, i.e. with a region of non-absorbing material between the active region and the reflecting surface
  • H01S5/166—Window-type lasers, i.e. with a region of non-absorbing material between the active region and the reflecting surface with window regions comprising non-semiconducting materials
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
  • H01S5/2036—Broad area lasers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
  • H01S5/204—Strongly index guided structures

Definitions

• the present invention relates to semiconductor laser diodes, particularly to AIGaAs-based laser diodes of high output power.
• Such laser diodes are commonly used in opto-electronics, often as so-called pump lasers for fiber amplifiers in the field of optical communication, e.g. for Erbium-doped fiber amplifiers.
• ridge waveguide laser diodes are suited to provide the desired narrow-bandwidth optical radiation with a stable light output power in a given frequency band.
• output power and stability of such laser diodes are of crucial interest.
• the present invention relates to an improved laser diode, the improvement in particular concerning the structure and design of the ridge waveguide; it also relates to a manufacturing process for such laser diodes.
• Semiconductor laser diodes of the type mentioned above have become important components in the technology of optical communication, particularly because such lasers can be used for amplifying optical signals immediately by optical means. This allows to design all-optical fiber communication systems, avoiding any complicated conversion of the signals to be transmitted, which improves speed as well as reliability within such systems.
• the laser diodes are used for pumping Erbium-doped fiber amplifiers, so-called EDFAs, which have been described in various patents and publications known to the person skilled in the art.
• EDFAs Erbium-doped fiber amplifiers
• An example of some technical significance are ridge waveguide laser diodes with a power output of 150 mW or more, whose wavelengths match the Erbium absorption lines and thus achieve a low-noise amplification.
• Several laser diodes have been found to serve this purpose well and are used today in significant numbers. However, the invention is in no way limited to such laser diodes, but applicable to any ridge waveguide laser diode.
• laser diode pump sources used in fiber amplifier applications are working in single transverse mode for efficient coupling into single-mode fibers and are mostly multiple longitudinal mode lasers, i.e. Fabry-Perot lasers.
• Two main types are typically being used for Erbium amplifiers, corresponding to the absorption wavelengths of Erbium: InGaAsP at 1480 nm; strained quantum- well InGaAs/AIGaAs laser diodes at around 980 nm.
• Semiconductor laser diodes of the types mentioned above have a number of problems.
• One particular significant one is that with increasing operating light output powers of vertically and laterally single mode semiconductor laser diodes, the maximum useable light output power is limited. It is believed that this is due to various reasons:
• the optical intensity typically increases towards the front facet.
• the latter has a reduced reflectivity compared to the back facet, due to the mirror coating:
• the coating of the front facet has a reflectivity between 0.1% and 10%, while the back facet coating has between 70% and 100%.
• Lang et al US Patent 6 014 396 disclose how to slightly broaden the ridge waveguide sections towards the front facet and the back facet to the same aperture. This reportedly reduces the series resistance compared to a standard narrow stripe device.
• the lateral gain regime is increased where the power density increases due to the asymmetric mirror coating of the front an back mirror.
• the effect of spatial hole burning is reportedly reduced when compared to a standard narrow stripe ridge waveguide.
• the chip pattern for the manufacturing process is designed such that adjacent laser diodes are arranged face to face. This arrangement however leads to problems in chip handling, laser diode characteristics, and reliability and is thus rather cumbersome.
• a process where all laser diodes are oriented in the same direction is by far preferable for the realisation of ridge waveguide like laser diodes.
• this invention improves the subject ridge waveguide laser diodes by shaping the ridge waveguide in a particular way.
• the focal point is the special arrangement of the waveguide broadening or flaring towards the front and/or the back facet.
• the ridge waveguide section is just slightly widened towards the front and/or the back facet, preferably to the same aperture. This reduces the series resistance compared to a standard narrow stripe diode laser if the effective contact area has been increased as compared to the latter.
• the lateral gain regime is increased where the power density increases.
• the beam can be further amplified, whereas in standard ridge waveguide laser diodes the amplification is locally saturated at lower power levels. The effect of spatial hole burning is thus reduced as compared to a standard narrow stripe waveguide.
• LIM Longitudinal Index Management
• the waveguide widening is small compared to common flared laser structures, usually below 10 ⁇ m; 2. front and back end-sections are ending in a locally straight waveguide geometry enabling an essentially standard, "narrow-stripe” manufacturing process; and 3. preferably front and back end-sections have the same aperture width or cross section.
• the invention concerns a semiconductor laser diode with an active region including a ridge waveguide having front and back facets.
• This ridge waveguide comprises at least three distinct segments along its longitudinal extension:
• the ridge waveguide has two tapered segments extending along the ridge waveguide, a first one widening towards the front facet and a second one widening towards the back facet of the waveguide, and two end segments, one associated with each facet.
• the two end segments have preferably the same cross section as the associated apertures.
• it may be called a "straight - flared - straight - flared - straight" waveguide design.
• the two tapered or flared segments have different lengths, in particular the first tapered segment being longer than the second tapered segment and they will be preferably of identical cross section.
• the two end segments may have substantially the same length, but this is not necessarily so.
• the ridge waveguide will be structured symmetrically with an essentially constant thickness along its whole length, the widening and the different cross sections being effected by a variation of the width of the ridge wave-guide.
• the center segment is of a first width, e.g. 3 ⁇ m wide, and the end segment of a twice said width, e.g. 6 ⁇ m wide.
• the ridge waveguide has a long center segment with a substantially constant first cross section, at least one flared segment widening towards one of the facets, and at least one short end segment between said tapered segment and said facet, said short end segment having a substantially constant second cross section larger than said first cross section.
• the ridge waveguide has two flared segments extending and widening from the center segment towards the facets in opposite direction, connecting the long center segment continously with said short end segments.
• a novel method for manufacturing a high power ridge waveguide laser diode with facets having identical cross sections forming mirror images enables strip-wise manufacturing with subsequent breaking of the laser diodes despite the flared or tapered structure of the ridge waveguide. This was impossible for flared designs until now.
• a ridge waveguide laser diode structured according to the present invention exhibits an extreme improvement over prior art laser diodes, especially with regard to its long-term stability and reliability.
• a further essential improvement can be achieved by combining the LIM design approach with a diode laser design termed "unpumped end section" and described in copending US patent application 09/852 994, entitled “High Power Semiconductor Laser Diode", which is incorporated herein by reference.
• the LIM - "unpumped end section” combination further enhances the reliability of the pump laser device. The reason is that the current density, as a possible cause for device degradation, is locally reduced at the sensitive front and back end section of the laser.
• the improvement needs just a minor increase in manufacturing complexity so that conventional manufacturing equipment may be used and usual manufacturing processes may be applied.
• the laser diode itself has the same dimensions as before, thus avoiding any packaging changes or problems.
• Fig. 1 shows an overview of the general structure of a ridge waveguide of a laser diode according to the invention
• Fig. 2 shows the ridge waveguides of three laser diodes in the manufacturing process according to the prior art
• Fig. 3 depicts two "consecutive" laser diodes in the manufacturing process according to the invention.
• Fig. 1 show the basic layout of a preferred embodiment of the invention which will be explained in detail in the following.
• the example shows the ridge waveguide of an AIGaAs 980nm pump laser diode in a top view.
• a semiconductor body here a not shown GaAs substrate, forms the basis for the laser diode.
• the laser diode consists essentially of a strained quantum well (SQW) active region sandwiched by two AIGaAs cladding layers.
• a top p-metallization also not shown, usually covers the semiconductor ridge waveguide together with some embedding material.
• the ridge waveguide layout shown in Fig. 1 is a of the "straight - flared - straight - flared - straight" type. It consists of five segments, starting from the front section on the right:
• a straight part i.e. a segment having a constant cross section or width, ending in the exit aperture of the laser diode.
• This straight segment is 6 - 8 ⁇ m wide and about 20 ⁇ m long in the present example.
• a flared part i.e. a segment having a decreasing cross section or width. Its width decreases from 6-8 ⁇ m to about 4 ⁇ m; its length is approximately
• a straight center part i.e. another segment having a constant cross section or width, smaller than the exit aperture of the laser diode.
• This straight segment is about 4 ⁇ m wide and about 1.2 mm, here precisely 1.116 mm long.
• Another flared part i.e. a second segment with changing cross section or width. Its width increases from about 4 ⁇ m to 6-8 ⁇ m; it is about 20 ⁇ m long. Thus it is significantly shorter than the first flared segment, i.e. has a significantly steeper opening angle.
• a third straight part i.e. a third segment having a constant cross section or width, ending in the back mirror of the laser diode. This straight segment is again 6-8 ⁇ m wide and about 20 ⁇ m long.
• the first straight part i.e. the segment having a constant cross section with the exit aperture of the laser diode, has a length of up to 5% of the total chip length.
• the first flared part i.e. the segment having a decreasing cross section or width
• the first flared part has a length of up to 30-60 % of the chip length . Its width decreases from the wide exit cross section to a fraction of the latter.
• the straight center part i.e. the second segment having a constant cross section/width, has a length of 40-70% of the chip length.
• the second flared part i.e. the second segment with changing cross section/width, has again a length of up to 5% of the total chip length. Its width increases from the cross section of the straight center part to the wide cross section of the exit aperture.
• the third straight part i.e. the third segment with a constant cross section or width, ends in the back mirror of the laser diode.
• the latter has preferably the same cross section as the exit aperture.
• This third straight segment has again a length of up to 5% of the total chip length.
• the LIM waveguide design improves the efficiency and linear light output power of the pump laser diode.
• the series resistance and therefore the junction temperature are reduced, resulting in improved reliability.
• the long straight center segment has waveguide properties comparable to a standard ridge waveguide section, while the adiabatically broadened front section, i.e. the right flared segment in Fig. 1 , further amplifies the zero order mode of the laser.
• the flared design principle itself is well known from semiconductor optical amplifiers. However, the application of this principle for a lateral single mode pump laser diode needs more than simple adjustments. It needs even changes to become suitable for a more or less standard ridge waveguide manufacturing process.
• the straight center segment and the adiabatically broadened segment towards the laser's output facet must be well designed to emit a lateral stable single-mode characteristic within the whole operating regime even. This also applies for laser diodes equipped with fiber Bragg gratings.
• the aperture width of the front facet and thus the width of the straight segment associated with the laser's output facet, should be limited to a maximum of about 8 ⁇ m for wavelengths of around 1 ⁇ m, i.e. 1000nm. Otherwise the coupling efficiency into a fiber tip with a standard lens is strongly reduced.
• the invention was first carried out on 980nm lasers in the AIGalnAs material system, it is also applicable to applications with other wavelengths, e.g. material systems ike InGaAsP/lnP or AIGalnAs/lnP at wavelengths ranging from 1300 ⁇ m to 1600 ⁇ m.
• Figs. 2 and 3 depict a top view of three laser diodes in the manufacturing process.
• Usually, several hundred laser diodes of the kind described here are fabricated on a semiconductor waver. Thus the manufacturing process and its details need great care.
• Fig. 2 shows a typical prior art design.
• flared ridge waveguide designs within a laser diode manufacturing process have been realized by designing a lithographic pattern which observed that the back segment of the waveguide was always straight, while the front region was flared.
• the chip pattern was designed such that adjacent laser diodes were arranged face to face as shown in Fig. 2. This arrangement however was found to result not only in problems in chip handling during manufacturing, but also in varying characteristics and low reliability of the laser diodes manufactured.
• Fig. 3 shows the novel design according to a specific aspect of the invention. Since all devices, i.e. laser diodes, are identical and oriented in the same direction, the new pattern results in equal devices all over the wafer as shown in Fig. 3. This design results in an improved device performance concerning maximum light output power and efficiency.

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• Physics & Mathematics (AREA)
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• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Optics & Photonics (AREA)
• Semiconductor Lasers (AREA)

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• 2002
  • 2002-04-24 US US10/131,335 patent/US6798815B2/en not_active Expired - Lifetime
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